1. Field of the Invention
The present invention relates to a laminated balun transformer used as a balanced-unbalanced converter for an IC of a cellular phone, an antenna of a television receiver, or other suitable devices.
2. Description of the Related Art
The balun transformer includes a first transformer 100 and a second transformer 200 as shown in an equivalent circuit diagram of FIG. 22. Then, coils 101 and 102 that define the first transformer 100 and coils 201 and 202 that define the second transformer 200 are connected as shown in FIG. 22 to thereby set the ratio of an impedance on a side of an unbalanced terminal 300 to an impedance on a side of balanced terminals 301 and 302 to about 1 to 4. Japanese Unexamined Patent Application Publication No. 04-206905 and Japanese Unexamined Patent Application Publication No. 06-120428, for example, describe a technology for a laminated balun transformer that has a circuit structure as described above. These laminated balun transformers are configured such that coil conductive patterns defining coils of the first and second transformers 100 and 200 or magnetic sheets and non-magnetic sheets are laminated. Thus, the size of the balun transformer is reduced.
However, the above described laminated balun transformers of the related art have the following problems. In the balun transformer, when the impedance of the unbalanced terminal side is set to about 1, the impedance of the balanced terminal side is the square of the number of transformers. For example, when the number of transformers is 1, the ratio of an unbalanced terminal-side impedance to a balanced terminal-side impedance is about 1 to 1. As shown in FIG. 22, when the number of transformers is 2, the ratio of an unbalanced terminal-side impedance to a balanced terminal-side impedance is about 1 to 4 (i.e., the square of 2). Thus, when the ratio of an impedance of a balanced terminal side to an impedance of an unbalanced terminal needs to be changed, it is necessary to change the number of transformers. However, the ratio of an impedance of the balanced terminal side to the impedance of the unbalanced terminal has discrete values, such as 1, 4, 9, . . . , and, therefore, the difference between the adjacent ratios is relatively large. Thus, when the line of the balanced terminal side to the line of the unbalanced terminal side has a ratio of impedance, such as 2, 3, 6, . . . , the existing balun transformer cannot be used. That is, the existing balun transformer is not versatile and cannot be used for lines having various impedances.